The hot gas content of fossil galaxy clusters

¹ Laboratoire AIM, IRFU/Service d’Astrophysique – CEA/DRF – CNRS – Université Paris Diderot, Bât. 709, CEA-Saclay, 91191 Gif-sur-Yvette Cedex, France
e-mail: gabriel.pratt@cea.fr
² CNRS, IRAP, 9 Av. colonel Roche, BP 44346, 31028 Toulouse Cedex 4, France
³ Université de Toulouse, UPS-OMP, IRAP, 31028 Toulouse Cedex 4, France

Received: 9 March 2016
Accepted: 23 April 2016

Abstract

We investigate the properties of the hot gas in four fossil galaxy systems detected at high significance in the Planck Sunyaev-Zeldovich (SZ) survey. XMM-Newton observations reveal overall temperatures of kT ~ 5−6 keV and yield hydrostatic masses M_500,HE ≳ 3.5 × 10¹⁴M_⊙, confirming their nature as bona fide massive clusters. We measure the thermodynamic properties of the hot gas in X-rays (out to beyond R₅₀₀ in three cases) and derive their individual pressure profiles out to R ~ 2.5 R₅₀₀ with the SZ data. We combine the X-ray and SZ data to measure hydrostatic mass profiles and to examine the hot gas content and its radial distribution. The average Navarro-Frenk-White (NFW) concentration parameter, ⟨ c₅₀₀ ⟩ = 3.2 ± 0.4, is the same as that of relaxed “normal” clusters. The gas mass fraction profiles exhibit striking variation in the inner regions, but converge to approximately the cosmic baryon fraction (corrected for depletion) at R₅₀₀. Beyond R₅₀₀ the gas mass fraction profiles again diverge, which we interpret as being due to a difference in gas clumping and/or a breakdown of hydrostatic equilibrium in the external regions. Our observations point to considerable radial variation in the hot gas content and in the gas clumping and/or hydrostatic equilibrium properties in these fossil clusters, at odds with the interpretation of their being old, evolved, and undisturbed. At least some fossil objects appear to be dynamically young.